1. Field of the Invention
The present invention relates to semiconductors and semiconductor packages. More particularly, but not limited thereto, it relates to the formation of interconnections between semiconductor substrates, such as semiconductor dice and adjacent substrates in a semiconductor assembly.
2. State of the Art
In conventional flip-chip attachment, an array of conductive bumps such as solder balls is formed on the surface of a semiconductor die, the conductive bumps being used to mechanically and electrically connect the die to higher-level packaging, such as a carrier substrate in the form of a printed circuit board. The formation of the solder balls may be carried out by a number of different methods. For example, a composite solder material of tin and lead may be electrodeposited through a mask to produce a desired pattern of solder masses to form bumps, the solder material then being heated to reflow to form solder balls by surface tension. Another technique is solder paste screening to cover the entire area of a wafer, the paste again being heated to reflow and form the solder balls.
U.S. Pat. No. 6,459,150 to Wu et al., issued Oct. 1, 2002, discloses an alternative technique for forming solder ball connections. A substrate, such as a die or interposer, includes a number of bond pads. Apertures are formed through the body of the substrate and each of the bond pads. A second substrate with corresponding bond pads is placed adjoining the first substrate and solder is deposited through the apertures. Solder reflow is then conducted to form solder balls that mechanically and electrically interconnect the substrates, while forming a conductive plug that passes through the body of the first substrate.
Each of these techniques thus requires solder reflow, typically conducted by passing the semiconductor structure through a reflow oven and subjecting the entire structure to the heat required to induce reflow. Such solder reflow usually involves four well-defined phases: preheat, soak, reflow (spike) and cool. First, in the preheat phase, the solder is warmed to a temperature that is just below its melting point. For example, for one conventional tin/lead solder composition, the structure may be heated to about 30° C. below a melting point of 183° C. In the soak phase, flux used to adhere the solder to under-bump metallization (UBM) formed on bond pads or redistributed bond pads is activated to remove any oxide on the metallization and the temperatures of the substrate and the solder are allowed to become more uniform and stabilized. During this soak period, the temperatures of the solder and the substrate are nearly constant or may increase slightly, for example, by about 20° C. In the reflow or spike period, the temperature is caused to increase rapidly and exceeds the melting point by between 20° C. and 50° C., such that the solder will melt, wet the metallized bond pads and assume a substantially spherical shape from the surface tension of the molten solder. Finally, in the cooling phase the solder balls and the substrate are allowed to cool to a temperature well below the melting point of the solder such that the solder balls solidify. In many instances, reflow to form the solder balls is followed by a subsequent reflow to connect the semiconductor die to a carrier substrate. Conventional techniques for flip-chip connection of a semiconductor die to a carrier substrate using solder are thus somewhat time consuming and subject the entire semiconductor die to elevated temperatures at least once for a substantial period of time, potentially subjecting it to damage as well as shortening the life thereof.
Techniques for flip-chip connection of semiconductor dice to carrier substrates using conductive materials other than solder are also known. For example, conductive or conductor-filled epoxies may be formed into discrete conductive elements in the form of columns or pillars and used to effect such mechanical and electrical connection. However, such an approach also involves preplacement of “dots” of the epoxy material on a semiconductor die or carrier substrate, assembly of the two electrical components desired to be attached, and then effecting cure of the epoxy. Furthermore, in many instances, it is desirable or even required to precure the epoxy to a tacky state, a so-called “B stage,” for ease of handling and preliminary adherence of the epoxy to a target surface prior to the final cure.
Accordingly, a method to enable flip-chip style attachment of a semiconductor die to a carrier substrate without the need for the time-consuming process of placing a solder paste or other solder ball precursor on a substrate followed by heating of the substrate or an assembly of substrates to form solder balls would constitute an improvement in the art, as would semiconductor packages constructed using such techniques. Additionally, such a method that would eliminate any need to preplace discrete conductive elements prior to assembly of a semiconductor die with another electronic component such as a carrier substrate would also be desirable.